1. Field of the Invention
This invention relates to viscous fluid coupling devices which serve as a drive for the radiator cooling fan of a vehicle engine, and more particularly, to such coupling devices which control the drive for the fan in response to ambient air temperature of the engine.
2. Description of the Prior Art
In the vehicle engine, it is normally necessary to increase the fan rotation thereby to cool the engine when ambient air temperature is high, and to decrease the fan rotation when ambient air temperature is low. In particular, when the engine is driven at a low temperature, it is desirable to prevent the fan from rotating in order to improve the warming characteristic of the engine.
In a conventional viscous fluid coupling device, however, there are drawbacks in that the engine is excessively cooled because no means is provided for preventing the fan from rotating, or that the construction of the device comes to be complicated even if the means is provided.